Dew point measuring apparatus



May 31, 1955 J, R, BQYLE 2,709,360

DEW POINT MEASURING APPARATUS Filed July 2, 1951 2 Sheets-Sheet 1 May 31, 1955 J. R. BoYLE DEW POINT MEASURING APPARATUS 2 Sheets-Sheet 2 Filed July 2, 1951 IIIIIIIIIIIIIIIIIII..

United States Patent DEW POINT MEASURING APPARATUS .iohn R. Boyle, Chicago, Laboratories, Inc., illinois 3 Claims. (Cl. 73-17) Ill., assignor to Illinois Testing Chicago, Ill., a corporation of The present invention relates to apparatus for determining the humidity or dew point of a gas. This gas may be substantially any gas containing a sensible amount of vapor or humidity, such as atmospheric air, conditioned air produced by air conditioning apparatus, industrial gases used in industrial operations, and numerous other gases of like nature.

The present application is divisional of my prior application Serial No. 504,348, tiled September 30, 1943, which issued on September 4, 1951, as Patent No. 2,566,307.

One of the objects of the invention is to provide an improved apparatus which will function through a much greater range of operating conditions than has been true of prior apparatus, such as a greater range of temperature, a greater range of moisture content, a greater range of pressure in the gas being tested or operated upon, etc.

Another object of the invention is to provide an improved apparatus which will enable the testing operation to be performed with greater facility, ease and certainty than has been possible with the prior apparatus heretofore known to me. Thus, the use of my improved apparatus does not demand a high degree of skill on the part of the operator.

Another object of the invention is to provide an improved apparatus which will have a greater degree of accuracy than has been obtainable with prior apparatus.

Another object of the invention is to provide an imand this application July 2, 1951,

proved apparatus in which a pressure change is produced in the gas sample as one of the fundamental steps of the operation, this pressure change bringing the gas to a temperature equivalent to the saturation point of its contained vapor or the point at which condensation occurs. This step of producing a pressure change in the sample to the end of condensing a small portion of the vapor to a condensate which can be readily sensed by visual observation or otherwise, is a distinctive feature which enables many of the foregoing advantages to be obtained.

.. of the inner circulating tube of the chamber 21. A gasket 24' composed of ber or like heat insulating material tends to thermally insulate the tube from the chamber. Extending upwardly from the tube 23 and head 24 is a liquid receptacle 25 adapted to contain the cooling liquid 26. Extending down centrally within the cooling tube 23 is a circulating tube 27 formed with en enlarged cup 28 at its upper end. A plurality of radially extending ns 29 projecting outwardly from the circulating tube 27 maintain the circulating tube 27 and cup 28 spaced from the cooling tube 23 and liquid storage receptacle 25 whereby a circulating flow of liquid can occur downwardly through the inner tube 27 and thence upwardly through the space 31 between the two tubes and into the liquid receptacle 25. The lowerend 27 is preferably deflected laterally, as indicated at 32, and this tube is formed with a lateral opening 33. This arrangement permits a thermometer 35 to be placed in the circulating tube in such position that the lower end of the thermometer normally reposes in the lower right hand corner of the cooling tube 23, in direct contact with the inner side of the tube wall at a point in closest proximity to the outer cooling surface 22. Thus, the thermometer 3S always affords an accurate measurement of the temperature of the cooling surface 22. In measuring the humidity or dew point of atmospheric air under most conditions the condensing surface 22 is maintained at the water temperature prevailing below floating ice, i. e. at maximum water density, which is substantially 39 F. Under these operating conditions, Water is the liquid medium 26 and lumps of ice 37 are placed inthe cup 28 for maintaining the water at or near the temperature of melting ice. It will be noted that a thermo-syphon circulation occurs constantly from the melting ice down through the inner circulating tube 27 and through the opening 33 into direct contact with the wall of the cooling tube 23 at the lower right hand corner 36 of said latter tube, the water thence flowing upwardly through the space 31 back into the liquid receptacle 25 and over the top of the cup 28 for again contacting the ice 37.

An electric light bulb 41 or other suitable source of light is arranged to project light rays against the cooling surface 22. In the illustrated embodiment, this light bulb is carried-in any suitable mounting socket 42 and the light rays are concentrated or focused on the condensing surface 22 by a lens 43 carried by a mounting ring 44, this ICC ` mounting ring having mounting on a boss portion 45 Other objects, features and advantages of the invention will appear from the following description of certain preferred embodiments thereof. In the accompanying drawings illustrating such embodiments:

Figure l is a diagrammatic side view of one embodiment of my invention, showing the condensing chamber in cross-section;

Figure 2 is a diagrammatic side View of another embodiment of my invention adapted to both pressure and suction operation of the condensing chamber, this View showing the valve apparatus arranged for pressure operation; and

Figure 3 is a view similar to Figure 2 showing the valve apparatus arranged for suction operation of this same embodiment.

Referring first ot the embodiment illustrated in Figure l, this construction comprises a condensing chamber 21 in which condensation of the moisture content is made to occur on a cooled condensing surface 22. This cooling surface is defined at the lower end of a cooling tube 23 which is carried by a head 24 closing the upper end v chamber 21. The bulb 46 or any other suitable through a suitable control projecting from the condensing 41 is energized from a battery source of electrical energy, switch 47.

The reflection of the light rays from the condensing surface 22 is viewed through an optical eye piece 4S which is mounted in an inclined tubular boss 49 extending from the chamber 21. The eye piece 48 comprises one or more lenses 51 of appropriate focal length and magnification for enabling condensate or dew on the surface 22 to be readily discerned. If desired, the eye piece 43 may be arranged for inward and outward focal adjustment in the tubular extension 49.

The air or gas which is to have its dew point measuredis forced into the condensing chamber 21 by a suitable pump 5S. This pump may be of any desired construction capable of creating pressures as high as or 150 pounds per square inch in the condensing chamber 21. The pump is preferably actuated by an operating lever 56 having any suitable operating connection with the pump piston 57 capable of creating the above pressures. In the exemplary arrangement illustrated in Figure l the piston rod 58 has its upper end pivotally connected with a forked inner portion of the operating lever 56 through the medium of a cross yoke and depending links 59. The inlet valve 61 of the pump has communication through a. ilexible inlet duct 62 with an inlet fitting 63. This inlet fitting prefarably includes a tine straining screen 64 through which all inlet air or gas must pass so as to minimize the likelihood of dust, dirt and other foreign matter getting into the pump or into the condensing chamber. All of the air or gas enters through the outer port 63 of the fitting. The inlet fitting 63 can be placed in any desired position, either for drawing atmospheric air when the atmospheric dew point is to be determined, or for introduction into ducts, chambers or other exploring uses. if it is desired to determine the dew point of a contained charge of gas, the inlet end of the flexible tube 62 may be coupled directly to the container in which the charge of gas is confined. The outlet valve 66 of the pump connects through pipe 67 with the interior of the condensing chamber 2l, this point of connection being preferably remote from the cooling surface 22 so as not to adversely aifect the desired temperature to be maintained at this cooling surface. ln the illustrated arrangement the pipe 67 opens into the upper portion of the condensing charnber 21 for impingement against the cooling tube 23 at a point opposite to the cooling surface 22, this arrangement tending to remove the heat of compression from the entering air or gas and also resulting in an increased thermo-syphon circulation upwardly through the cooling tube 23. A pressure gauge 69 is connected with the interior of the condensing chamber 21 to give an accurate indication or measure of the pressure prevailing therein, this preferably being a Bourdon tube gauge, although other types of gauges may be employed. A discharge valve 7l is provided for discharging the condensing chamber 2l to atmosphere after a measurement has been completed.

Referring now to the operation of this lirst embodiment of the invention we will assume that it is desired to determine the dew point of the atmosphere or of a particular gas where such atmosphere or gas is at a temperature below 39 F.

The temperature of 39 F. is arbitrarily chosen because that is substantially the water temperature prevailing below melting ice and hence when the sample of air or gas to be measured has a temperature below 39 F. l am enabled to use melting ice for establishing the temperature of the condensing surface 22. The liquid chamber 25 and cup 28 are filled with water and ice approximately to the levels indicated and the unit allowed to stand for a sufficient interval to insure that the condensing surface 22 has reached the temperature of approximately 39 F. This can be checked or watched through the thermometer 3S. Thereupon the pump 55 is operated to force the air or selected gas into the condensing chamber 2. During the first part of this pumping interval it may be desirable to leave the discharge valve 71 open so as to discharge from the chamber the residual air or gas remaining therein from the preceding measuring operation. After such venting of the old air 1 the density of or gas has been completed the valve 71 is closed and the light 4l illuminated, and the operator then begins a continuous observation of the condensing surface 22 through the eye-piece 48. It will be evident that as the pump 55 continues to force more air or gas into the condensing chamber, with consequent increase of pressure therein, it also increases the density ot the moisture or vapor particles in the chamber. When the density of the vapor reaches the saturation point of air at 39 F. then condensation immediately forms on the condensing surface 22 because this surface has a sustained temperature of 39 F. More specifically, this condensation `vill occur when the density of the vapor in proximity to the surface 22 has reached the approximate value of m2103943 pound per cubic foot or equivalent to 2,760 grains per cubic foot as obtained from the standard saturated steam tables, where the vapor pressure corresponding to 39 F. is 0.1171 pound per square inch, this being the saturation point or dew point of air at 39 F. Once the saturation point or dew point is reached the condensation forms quickly on the surface 22 and is immediately visible through the eye-piece 4S. Prior to the condensing of the vapor on the surface 22 only a relatively small amount of light is reliected up to the eyepiece 4S. The collection of condensate on this surface 22 immediately improves the reiiecting characteristic of the surface by reason of the multitudinous tiny globular or spherical surfaces which these moisture particles present to the light rays coming from the light 4l. These minute globular surfaces reilect a greatly increased amount of light up to the eye-piece 48 as soon as condcnsation occurs. The angle o incidence of the rays from the light bulb and the angle of reflection up to the eye-piece 42 are preferably arranged to augment as much as possible the contrast between the absence of condensation and the presence of condensation on the surface 22. Another source of contrast can be provided for by extending a ange 23 down from the lower end of the tube 23 to provide a lower contrasting surface 22' immediately below the condensing surface 22. The contrasting surface 22' does not receive as much cooling affeet from the circulating ice water as does the condensing surface 22, and hence contrast of appearance between the two surfaces also helps to establish the fact that condensation has occurred on the surface 22. As soon as this condensation occurs the operator ceases actuating the pump 55 and immediately notes the pressure which is indicated by the pressure gauge 69. This pressure in its relation to atmospheric pressure or to absolute pressure is a function of the dew point of the sample of air or gas contained within the condensing chamber 21. The dew point may be obtained from a direct reading calibration of the pressure gauge 6 9, or it may be obtained by reference to a chart or table. Where the instrument is intended always to operate under the same conditions, as for example when the condensing surface 22 is always cooled to the temperature of water below melting ice, then the pressure gauge 69 can be directly calibrated in terms of the dew point of the air or gas in the condensing chamber. However, if other mediums are to be employed to maintain a predetermined temperature at the condensing surface 22 then it may be preferable to take the pressure readings of the pressure gauge 69 and arrive at the dew point by mathematical computation or by reference to charts or tables. To illustrate the manner of arriving at the dew point by mathematical computation let us assume that at the instant of condensation the pressure reading of the gauge 69 was 147 pounds, i. e. l0 times that of the average sea level barometric pressure of 14.7 pounds, Hence, the vapor in the same being pumped into the condensing chamber was only one tenth of the vapor density of saturated air at 39 F. Since the vapor density of saturated air at 39 F. is 2.760 grains per cubic foot, then the density of the sample being pumped into the condensing chamber was 1,-10 of this value or approximately .276 grain per cubic foot. Referring now to standard tables of saturated water vapor, it will be seen that .276 grain per cubic foot represents the weight of saturated vapor in air of --l0 F. Hence, the air or gas in the condensing chamber has a dew point of 10 F.

When using melting ice as the cooling medium for cooling the condensing surface 22, it is also possible to measure the dew point of gases having higher dry bulb temperatures than 39 F., provided that the humidity of such gases is below the saturation point of 39 F. air, i. e., below 2.760 grains per cubic foot. For example, it is possible to measure the dew point of air having a 70 F. dry bulb temperature when said air has a relative humidity of less than substantially 34%, viz., less than 2,760 grains of vapor per cubic foot.

When measuring air or gas having an extremely low dew point it may be desirable to employ lower ternperature cooling mediums than melting ice for cooling the condensing surface 22. For example, when measuring a gas having a moisture content less than 3 or 4 grains of vapor per pound of dry air, it is necessary to create pressures in the condensing chamber 21 ranging upwards of 150 pounds per square inch, and these pressures may be objectionable. Accordingly, I may substitute Dry Ice or Freon or a like medium in place of melting ice in the cooling tube 23. An appropriate thermometer 35 is preferably employed in conjunction with either of these other cooling mediums for giving an accurate indication of the temperature of the cooling surface. When using any one of these other cooling mediums the operation is substantially the same as described above. Having a known temperature at the condensing surface 22 and a known pressure of the air or gas at the instant of condensation, it is possible to determine the dew point of such air or gas by a mathematical computation or by reference to appropriate tables or charts, as above described.

When measuring the humidity of a vapor containing gas or air having a higher humidity than 2.760 grains per cubic foot, I contemplate substituting higher temperature cooling mediums in place of the melting ice. For example, carbon tetrachloride, because of its relatively high rate of evaporation, will maintain the cooling surface 22 at approximately 65 F. Furthermore, it is also possible to maintain a circulatory ow of water of known temperature down through the circulating tube 27 and up through the cooling tube 23, or to maintain a fixed quantity of water in these tubes, in either situation utilizing a thermometer 35 to keep a close check on any variations of temperature of the condensing surface 22. If the pressure gauge 69 is calibrated for the 2.760 grains corresponding to 39 F. dew point air, then the use of water at substantially 5 81/2" F. will result in all gauge readings being raised by a scale multiplier of 2, (Viz., 2.76 2=5.52, such being the grains of vapor per cubic foot at the saturation point of 581/z F. air). Similarly, for a scale multiplier of 3 we can use water of approximately 71 F. (viz, 2.76 3=7.28, such being the grains of vapor at the saturation point of 71 F. air). In the same manner, the use of Water of 70 F. temperature would result in the use of a scale multiplier of 2.92.

Referring now to the embodiment illustrated in Figures 2 and 3, this construction employs a condensing chamber 21 substantially the same as that above described, but provision is made in this modified construction for operating such condensing chamber either under compression or under evacuation. Inasmuch as the condensing chamber is substantially the same as that above described there is no need of repeating the description, the same reference numerals being shown as applied to the same parts. This modiiied construction comprises additional elements consisting of a reservoir 81, a heatdissipating or temperature equalizing coil 82, and a system of piping and valves which enables the same unit to be employed alternatively for pressure operation or for vaccum operation. The reservoir 81 is shown as functioning as a supporting base on which the condensing chamber 21 and pump 55 are mounted, although this is not essential. The ilexible inlet duct 62 leading from the inlet fitting 63 has connection through valve A with T connector 84. Leading from one branch of this T connector is a pipe or duct 85 extending to the inlet valve 61 of the pump 55. The other branch of the T connector 84- has connection through pipe or duct 86 with the upper portion of the condensing chamber 21. A valve B is interposed in the pipe 86. Leading from the outlet valve 66 of the pump 55 is the coil 82 which is adapted to dissipate heat or equalize the temperature of the sample of air or gas being measured, as will be hereinafter described. The other end of the coil 82 opens into a T connection 87 which has its two branches connected respectively with valves A1 and B1. The valve A1 6 pipe 88 with the interior of the reservoir 81. The valve B1 opens directly to atmosphere. A conduit or passageway 89 leads from the interior of the reservoir 81 into the interior of the con densing chamber 21. The four valves A, A1, and B, B1 may be any desired form of shut-oit Valves, being preferably quick opening and quick closing cocks, however. The valves A and A1 are adapted to be operated conjointly, and may therefore be linked together mechanically if desired; and, similarly, the Valves B and kB1 are adapted to be operated conjointly and may also be mechanically linked together.

When the apparatus is to be operated by the pressure methods described above in connection with the preceding embodiments, the valves A and A1 are both moved to their open positions and the valves B and B1 are both moved to their closed positions, as diagrammatically illustrated in Figure 2. When the pump 55 is now actuated, the sample of air or gas is drawn in through the ilexible inlet duct 62 and valve A to the inlet side of the pump 55. From the outlet side this air or gas is thence forced through the coil 82 and through valve A1 down into-the reservoir 81. Passing the air or gas through the coil 82 serves to dissipate the heat of compression or to elect a substantial equalization between the temperature of the air or gas being measured and atmospheric temperature. The reservoir 81 is preferably composed of metal, and has a large heat radiating surface. Hence, this reservoir also serves to dissipate heat or equalize the temperatures. From the reservoir 81 the compressed air or gas passes upwardly through conduit 89 into the condensing chamber 21. The outlet valve 71 may be opened during the initial operation of the pump 55 for scavenging old gas from the condensing chamber 21 and the reservoir 81 at the start of a new operation. Upon closing this valve 71 the pressure begins to build up in the condensing chamber 21 in the same manner described above in connection with Figure l. Thereupon the apparatus is operated by the pressure method in the same manner as described above, using any one of the aforementioned mediums for cooling or establishing a desired temperature at the condensing surface 22. As mentioned above, the provision of the coil 82 serves to dissipate the heat of compression in this pressure method of operation of the unit. The provision of the reservoir 81 also aids in dissipating the heat of compression, and it further tends to equalize or render more uniform the constituency of the sample of gas being measured.

Referring now to Figure 3, this same apparatus can also be converted to the vacuum or suction mode of operation by merely closing the valves A, A1 and opening the valves B, B1. It will now be seen that when the pump 55 is actuated the suction side of the pump is connected through valve B with the interior of the condensing chamber 21, thereby tending to evacuate the condensing chamber. The outlet side of the pump 55 is merely discharged to atmosphere through coil 82 and valve B1. This evacuation or pressure reduction method is particularly intended for situations where the sample of gas being measured has a dew point which is above the temperature of the condensing surface 22. This sample of air or gas is admitted to the condensing chamber 21 through the valve 71, which may be connected with a conduit or exploring tube if controlled or selective sampling is desired. Assuming that the sample has a dew point which is above the temperature of the condensing surface 22, it will be apparent that condensate or dew will show on the surface 22 as soon as the sample enters the chamber 21. If the sample is at atmospheric pressure, such inflow is induced by operating the pump 55. Thereupon the valve 71 is closed and the pump continued in operation until the pressure in the chamber 21 has been brought down to the point where the dew disappears from the surface 22. Following this the pressure in the chamber is slowly increased by a gradual opening has connection through of the valve 71 until the dew reappears. This pressure Ps is saturation of the vapor at temperature t, the same as before.

This pressure at which the dew disappears and reappears is the saturation of the vapor at the temperature of the condensing surface 22, the same as before. Let us assume an example where ice water is being used to maintain the condensing surface 22 at a temperature of approximately 39 F. (having the aforementioned vapor saturation of 2.76 grains per cubic feet) and that the prevailing barometric pressure is 29.92 inches of mercury. It will also be assumed that the sample to be tested has a temperature of 80 F., and that it is necessary to evacuate the condensing chamber down to 20 inches of mercury absolute pressure before reaching the critical point where condensation disappears and reappears on the surface 22. The grains of vapor per cubic foot in this sample are then derived mathematically as follows:

20 Xara-4.13 grains per cubic foot. Referring now to standard steam tables, this vapor density represents saturation at 50.2 F., and hence it will be seen that the sample has a dew point of 50.2 F.

By reason of being able to operate under either pressure or suction, the embodiment of Figures 2 and 3 can perform measuring operations over a wide range of temperatures. For example, the apparatus can be readily constructed to withstand 150 pounds per square inch for pressure operation, and the pump 55 can be readily constructed to draw a suction down to 5 inches of mercury absolute pressure for suction operation. Assuming that the condensing surface 22 is maintained substantially at the 39 F. temperature referred to above, the 150 pound maximum pressure wiil enable dew point measurements to be carried down to a lower limit of approximately 11 F. dew point. This is arrived at as follows:

15 (approximate atmospheric pressure) V 2 76: 270

150 (maximum pressure) A grains per cubic foot. This is the weight of saturated vapor at approximately 11 F., such being therefore the lower limit of operation at 150 pounds pressure and a 39 F. constant temperature at the condensing surface. The upper limit under a depression of 5 inches of mercury is approximately a dew point of 93 F. This is arrived at as follows:

0. sc t 2L 92 barometric pie si re) Xgjzll 5 (maximum suction) grains per cubic foot. This is the weight of saturated vapor at approximately 93 F., such being therefore the upper limit of operation at a suction of 5 inches of mercury and a constant temperature of 39 F. at the condensing surface 22.

For performing dew point measuring operations above 93 F. or below l 1 F. other fluids may be used for predetermining the temperature of the condensing Vsurface 22, such as a volatile uid with fixed atmospheric boiling point, or warm water with a constant checking of the temperature indicated by thermometer 35.

When using a medium like melting ice to predetermine the temperature of the condensing surface (at substantially 39 F), there are enough constants that a pressure ratio gauge scale can be arranged to read directly in grains per cubic foot. Also, a simple secondary gauge scale can be arranged to read directly in terms of dew point. Where otter constant temperatures, in addition to 39 F., are to be employed, additional gauge scales may be added for these additional temperatures. For greater accuracy of gauge indication, it may be desirable to employ two gauges, one for pressure and one for vacuum; or one for `high pressure and vacuum and one for low pressure and vacuum, the latter being closed otl. during high pressure operation. I also contemplate employing a thermocouple placed against the condensing surface 22 for a greater accuracy of temperature measurements. It will be understood that the surfaces 22 and 22 are preferably highly polished to accentuate the appearance of the dew thereon. In Figures 2 and 3 the valve 71 and the upper end of the conduit S9 are positioned near the upper end of the cooling tube 23 so that whenever air enters the condensing chamber 21 through said valve or conduit the tube will exert a cooling influence on the air and the thermosyphon flow through the tube will also be augmented. In some instances the outer and inner tubes 23 and 27 may be made considerably shorter than shown to obtain the desired temperature at the surface 22. The pump 55 may be actuated by the lever 56 or by a knob 56 secured to the upper end of the piston rod 5S.

While I have illustrated and described what l regard to be the preferred embodiments of my invention, nevertheless it will be understood that such are merely exemplary and that numerous modifications and rearrangements may be made therein and in the method of carrying the invention into effect without departing from the essence of the invention.

I claim:

l. In apparatus for measuring the dew point of a gas, the combination of a condensing chamber, means for introducing the gas to be tested into said chamber, a cooling tube having a closed end extending into said chamber and sealed thereto, said cooling tube being provided with a condensing surface in said chamber, there being a wall surrounding the upper open end of said cooling tube for retaining liquid, a second tube within said cooling tube, an ice receptacle connected to the upper end of said second tube within said wall for circulating ice water through said tubes by thermo-syphon flow for delivering water of maximum density into Contact with said condensing surface, a pump for increasing the pressure of the gas in said chamber to cause dew to collect on said condensing surface, means for projecting light rays against said surface, a sight window for observing the formation of dew on said surface by variation in the reection or refraction of the light rays caused by the dew,

and a pressure gauge for determining the pressure pre-` vailing in said chamber at the time the dew forms on said surface.

2. In apparatus for measuring the dew point of a gas, the combination of a condensing chamber, a reservoir, a gas inlet, and a pump all cooperating for introducing the gas to be tested into said condensing chamber, a convertible system of piping and valves connected with said condensing chamber, reservoir, gas iniet and pump whereby in one setting of said valves said pump is operative to cause said gas to be pumped through said reservoir and into said condensing chamber at a pressure above atmospheric pressure and whereby in another setting of said valves said pump is operative to cause said gas to be maintained in said condensing chamber at a pressure below atmospheric pressure, a condensing surface in said condensing chamber upon which dew is adapted to collect from the moisture contained in the gas, pressure responsive means for indicating the pressure prevailing in said condensing chamber and means for sensing the presence or absence of dew on said condensing surface.

3. In apparatus for measuring the dew point of a gas, the combination of a condensing chamber, means for introducing the gas to be tested into said chamber, a cooling tube having a closed end extending into said chamber and sealed thereto, said cooling tube being provided with a condensing surface in said chamber, there being a wall surrounding the upper open end of said cooling tube for retaining liquid, a second tube within said cooling tube, an ice receptacle connected to the upper end of said second tube within said wall for circulating ice water through said tubes by thermo-syphon ow for delivering water of maximum density into contact with said condensing surface, means for changing the pressure of the gas in said chamber to cause dew to co1- lect on said condensing surface, means for projecting light rays against said surface, a sight window for observing the formation of dew on said surface by variation in the reection or refraction of the light rays caused by the dew, and a pressure gauge for determining the pressure prevailing in said chamber at the time the dew forms on said surface.

References Cited in the iile of this patent UNITED STATES PATENTS Scott Feb. 6, 1934 Deaton et al. Apr. 28, 1942 OTHER REFERENCES 

1. IN APPARATUS FOR MEASURING KTHE DEW POINT OF A GAS, THE COMBINATION OF A CONDENSING CHAMBER, MEANS FOR INTRODUCING THE GAS TO BE TESTED INTO SAID CHAMBER, A COOLING TUBE HAVING A CLOSED END EXTENDING INTO SAID CHAMBER AND SEALED THERETO, SAID COOLING TUBE BEING PROVIDED WITH A CONDENSING SURFACE IN SAID CHAMBER, THERE BEING A WALL SURROUNDING THE UPPER OPEN END OF SAID COOLING TUBE FOR RETAINING LIQUID, A SECOND TUBE WITHIN SAID COOLING TUBE, AN ICE RECEPTACLE CONNECTED TO THE UPPER END OF SAID SECOND TUBE WITHIN SAID WALL FOR CIRCULATING ICE WATER THROUGH SAID TUBES BY THERMO-SYPHON FLOW FOR DELIVERING WATER OF MAXIMUM DENSITY INTO CONTACT WITH SAID CONDENSING SURFACE, A PUMP FOR INCREASING THE PRESSURE OF THE GAS IN SAID CHAMBER TO CAUSE DEW TO COLLECT ON SAID CONDENSING SURFACE, MEANS FOR PROJECTING LIGHT RAYS AGAINST SAID SURFACE, A SIGHT WINDOW FOR OBSERVING THE FORMATION OF DEW ON SAID SURFACE BY VARIATION IN THE REFLECTION OR REFRACTION OF THE LIGHT RAYS CAUSED BY THE DEW, AND A PRESSURE GAUGE FOR DETERMINING THE PRESSURE PREVAILING IN SAID CHAMBER AT THE TIME THE DEW FORMS ON SAID SURFACE. 